The present invention relates to an apparatus such as a mail sorter for separating and classifying a plurality of letters and post cards (hereinafter sometimes referred to as the letters) in conveyance, or more in particular to an overlap detection apparatus for detecting a plurality of letters overlapped in conveyance.
In an apparatus for detecting overlapped letters in conveyance disclosed in JP-A-4-266349, at least a roller is applied to the letters in conveyance and thus the side of the letters in contact with the roller is delayed, so that an overlap is detected according to whether the length of the letters undergoes a change.
On the other hand, JP-A-7-172628 discloses a method of detecting an overlap from the upward warping of the tail end of the letter released from a guide by which it is deformed.
Further, according to the conventional methods described in JP-B-60-30617 and JP-B-60-30618, a space is formed by separating an outer belt from an inner belt at a bent portion of a conveyance path, and two light projectors/detectors located at different positions project light on the letters passing through the bent portion, thereby detecting an overlap according to whether the light is shut off by the forward end of a letter overlapped on the middle part of another letter.